This invention concerns lip rolling or curling machines which are used to form a lip on the rim of thermoformed plastic containers such as disposable cups to provide a formed edge around the cup rim. Such machines have been known and used successfully for many years, as described in U.S. Pat. No. 3,337,919 issued on Aug. 29, 1967 for a "Container Rim Bending Apparatus". This type of machine includes a set of feed rollers which are arranged in a circular array with a stack of nested cups directed into the space between the feed rollers to be engaged by the rollers to rotate the stack and feed it forward at a predetermined rate through an oven and thence into a set of rotating curling screws.
The curling screws comprise a set of helically grooved rotary members, the grooved curling screws each configured to cooperate with the other curling screws in the set to roll form each cup rim as they drive each cup through the circular space between the curling rollers.
The spacing of curling screws is sometimes designed to be adjustable to be adapted to different cup sizes. The rotation of each of the curling screws must be accurately synchronized with each other in order to properly form the lip, and a timing belt drive is typically used to achieve this. Thus, any adjustment must not affect the timing belt engagement, or the synchronism must be carefully reset. In the past, each curling screw was mounted to be separately adjustably positioned along a radial line from the center of the spacing circle to be located either further in or further out to match the circular space to the particular cup size being processed. There are disadvantages to this individual adjustment, including the fact that considerable effort and time are required to position each individual curling screw accurately, requiring shut down of production while the adjustment is carried out. One or more of the curling screws are typically located in difficult to reach locations within the machine, which contributes to the difficulty and time required for an adjustment. A four curling screw machine is superior to one having three curling screws, as four curling screws are better able to exert compression on the cup rim during forming than three screws, since opposing curling screw pairs may thus be provided. However, the difficulties encountered in making set up adjustments on individual curling screws has resulting in most machines having only three curling screws.
Another important disadvantage is the fact that the center axis around which the curling screws are arranged is usually shifted somewhat during adjustments such that the center axis is not aligned with the center of the space between a set of guide rods used to guide the cup stack into the space between the curling screws. This discrepancy as well as other misalignments limits the speed at which cups can be fed through these adjustable machines.
The cups are heated in an oven mounted just ahead of the curling screws, to soften the cup rims preparatory for the lip rolling operation. Thus, the curling screws must be cooled to prevent being heated excessively by contact with the cups and to maintain a proper forming temperature of the cups so that the lips will be set after exiting the machine.
This cooling is usually carried out by the circulation of a cooling liquid such as water directly within each curling screw, as it has heretofore been thought necessary to have direct contact of the coolant with the curling screws to obtain adequate cooling. The disadvantage of this approach is that when the curling screws are replaced, as for processing a different family of container sizes, or when being serviced, the coolant leaks out, creating a mess to be cleaned up, making replacement a lengthy process.
It is the object of the present invention to provide an adjustable curling screw mechanism for a lip rolling machine which allows a quick, convenient, and simultaneous adjustment of the radial position of all of the curling screws in a manner which accurately maintains the location of the center axis around which the curling screws are arranged, and also does not interfere with the timing belt engagement.
It is a further object of the present invention to provide a coolant circulation path within the curling screws which precludes spillage of coolant when the curling screws are replaced.